1. Field of the Invention
This invention relates to a sensor for use in an indicator to provide a warning of exposure to a toxic gas, and to a method of producing such a sensor.
2. Description of Related Art
Personal badge-type exposure indicators are critical components of next generation protective gear. Ideally, such indicators not only warn of an exposure event but also quantify the extent of exposure and provide a stream of data in real time so that informed decisions can be made regarding ambient toxicity.
The inventors have determined that a film of naked nanoparticles on a non-conductive substrate such as glass or polyethylene is a suitable sensor for use in an indicator of the type for use with protective gear.
The current flow between metal nanoparticles interconnected by molecules is a fundamental process underlying single electron transistors and much of the field of molecular electronics. When the distance between nanoparticles is greater than 2 nm and the barrier to charge transfer greater than 1 eV, current flow between particles occurs via single-electron tunneling. Under these conditions, the residence time of the electron on a nanoparticle is relatively long and electric current flow occurs via a series of discrete tunneling “hops” of electrons from nanoparticle to nanoparticle. In this regime, the rate of current flow depends on a number of factors including the bias applied, the electronic structure of the interparticle molecules, the goodness of the electrical contact between the molecules and the surface of the nanoparticles, the distance between nanoparticles and the charging energy of the nanoparticles.
Current flow through monolayers of close-packed metal nanoparticles have been extensively studied. Examples studied to date include films of thiol-capped 2.7-4.8 nm diameter Ag nanoparticles, and monolayer-protected gold nanoparticles. The nanoparticles in such films are typically encapsulated in monolayer coatings, which prevent particle coalescence as well as retain a constant and well defined interparticle spacing. The formation of films from the coated nanoparticles occurs via self-assembly. The resulting bilayer of molecules between the nanoparticles in such films provides a barrier to direct charge transport between particles, ensuring that interparticle, single-electron tunneling of charge across the molecular bridge between the nanoparticles is the dominant charge transfer mechanism. In this configuration, the conduction characteristics of the nanoparticle film are expected to be especially sensitive to the nature of the molecular bridge. Self-assembly methods, however, are not ideally suited for study of the molecular bridge because changing the type of bridge also changes the interparticle spacing so the results are convoluted.